Biodegradable polymers which decompose in the natural environment have been attracting attention from the viewpoint of the preservation of the global environment, and various biodegradable polymers have been developed. Out of these, polylactic acid has high transparency, can be melt molded, can be manufactured economically from biomass by fermentation using a microorganism and is expected to be used as an optical material.
Along with the expansion of the display market, demand for more clear images is growing and a material which has not only transparency but also higher optical properties is desired.
In general, birefringence occurs in polymers due to a difference in refractive index between the direction of the molecular main chain and a direction perpendicular to that direction. Birefringence must be controlled precisely according to application purpose and needs to be small in the case of a polarizing plate protective film which is used in a liquid crystal polarizing plate.
A triacetyl cellulose (TAC) film has been often used as the polarizing plate protective film. Along with the spread of displays such as large-sized liquid crystal displays and plasma displays, required films have been becoming large in size and the need to reduce a birefringence change distribution has been becoming large. Therefore, a material which has a small change in birefringence caused by external force, excellent dimensional stability at the time of heating and a small change in birefringence caused by heat stress is desired. That is, an optical material which has a small photoelastic coefficient and a small heat shrinkage factor is desired.
There are known TAC homopolymer and methyl methacrylate homopolymer (PMMA) as optical materials having a small photoelastic coefficient. There is also known amorphous polyolefin (APO) (non-patent document 1). However, these materials have a problem such as a large change in birefringence caused by external force or too low polarity.
Further, a material comprising acrylic resin and polylactic acid is proposed as an optical material having a small photoelectric coefficient. A film made of this material has a photoelectric coefficient of more than −13×10−12/Pa and less than 12×10−12/Pa (patent document 1). However, this material has large heat shrinkage by heating and a disadvantage that, when it is used as a polarizing plate protective film, the difference in birefringence changes by the generated heat stress. Therefore, the development of a material which has a small photoelastic coefficient, a small change in the difference of birefringence caused by external stress, excellent dimensional stability at the time of heating and suppresses the generation of heat stress is awaited.    (non-patent document 1) Introduction to Chemistry No. 39, 1998 (published by the Academic Society Publishing Center)    (patent document 1) JP-A 2006-227090